Following gaining a successful resolution to this issue using the 'Screen' mode of the Opacity function, I have now encountered a slightly different issue. Having layered multiple frames, I am experiencing an awful lot of what I think you might call 'noise'. The more I layer the frames of black, the more I lose the pure nice black colour of the night sky and it becomes lighter and more blurry and grey. Is there a color correction or other such function or technique that might eliminate that noise and maintain the clean black color?

Thanks for taking the time. I'll try to explain. So I took a continuous frame of some lights in the dark on the skyline that looked like this...

The frame moves around so they're in a different position on the screen, so I chopped it up and isolated those moments before layering a few frames and adding 'Screen' opacity to it. Effectively therefore the lights appear numerous times in the frame. Here's one of about 5 layers...

Effectively I wish to have these lights all fan out and fill the screen but as you can see the image is becoming grainier and grainier the more I layer and nest the frames. Here's what it looks like after 10...

Not pretty I'm sure you'll agree. I hope you can see what I'm trying to do though. I'm no expert but presumably there is some sort of technical term for the way that the multiple layers of black are obscuring the quality of the image. It's not your average difficulty I admit but if you know of any method by which the body of the image can be restored to that nice dark black whilst the lights remain clear and sharp then you would rescue me from an ongoing frenzied Googling session.

Back to your goal. In your previous thread you was already proposed to use Luma Key. You can definitely try to accomplish your task in PrPro. However, as Joe points out, you're better off with After Effects, 'cos it is a compositor and, hence, designed to do multiple layered compositing much more convenient.

Thanks very much for your advice. Looks like I'm gonna have to get to grips with AE. In the process of learning- any chance you you could give me a quick explanation of how you created that layering effect?-just a briefly outline of the processes I should be trying to learn..?

Also I downloaded Unmult and copied it into the effects folder but its not showing up when I search my Effects- what have I done wrong? Is that the correct way to import effects?